The present invention is generally directed to organic microelectronic devices, and more specifically, in embodiments to the use of a class of polythiophenes as active materials in thin film transistors. The polythiophenes selected can be comprised of repeating thienylene units in which only certain thienylenes possess side chains, and which thienylene units are arranged in a regioregular manner on the polythiophene backbone.
The polythiophenes are in embodiments substantially stable enabling their device fabrication to be accomplished at ambient conditions, and wherein the devices provide higher current on/off ratios, and are operationally more stable as their performance usually does not degrade as rapidly as those of known regioregular polythiophenes such as regioregular poly(3-alkylthiophene-2,5-diyl). More specifically, the polythiophenes of the present invention contain in embodiments repeating segments of 3,4-disubstituted-2,5-thienylene units flanked by unsubstituted 2,5-thienylene units and an optional divalent linkage. The side chains assist in inducing and facilitating molecular self-organization of the polythiophenes during film fabrication, while the unsubstituted thienylene units and the optional divalent linkage, which have some degree of rotational freedom, can disrupt the extended xcfx80-conjugation along the polythiophene chain, thus suppressing its propensity towards oxidative doping.
Semiconductive polymers like certain polythiophenes, which are useful as active semiconductor materials in thin film transistors (TFTs), have been reported. A number of these polymers have reasonably good solubility in organic solvents and are thus able to be fabricated as semiconductor channel layers in TFTs by solution processes, such as spin coating, solution casting, dip coating, screen printing, stamp printing, jet printing, and the like. Their ability to be fabricated via common solution processes would render their manufacturing simpler and cost effective as compared to the costly conventional photolithographic processes typical of the silicon-based devices such as hydrogenated amorphous silicon TFTs. Moreover, desired are transistors fabricated with polymer materials, such as polythiophenes, referred to as polymer TFTs, include excellent mechanical durability and structural flexibility, which may be highly desirable for fabricating flexible TFTs on plastic substrates. Flexible TFTs would enable the design of electronic devices which usually require structural flexibility and mechanical durability characteristics. The use of plastic substrates, together with an organic or polymer transistor component, can transform the traditionally rigid silicon TFT into a mechanically more durable and structurally flexible polymer TFT design. The latter is of particular appeal to large-area devices, such as large-area image sensors, electronic paper and other display media as flexible TFTs, could enable a compact and structurally flexible design. Also, the selection of polymer TFTs for integrated circuit logic elements for low-end microelectronics, such as smart cards and radio frequency identification (RFID) tags, and memory/storage devices may also greatly enhance their mechanical durability, thus their useful life span. Nonetheless, many of the semiconductor polythiophenes are not stable when exposed to air as they become oxidatively doped by ambient oxygen resulting in increased conductivity. The result is larger off-current and thus lower current on/off ratio for the devices fabricated from these materials. Accordingly, with many of these materials, rigorous precautions have to be undertaken during materials processing and device fabrication to exclude environmental oxygen to avoid oxidative doping. These precautionary measures add to the cost of manufacturing, therefore, offsetting the appeal of certain polymer TFTs as an economical alternative to amorphous silicon technology, particularly for large-area devices. These and other disadvantages are avoided or minimized in embodiments of the present invention.
A number of organic semiconductor materials has been described for use in field-effect TFTs, which materials include organic small molecules such as pentacene, see for example D. J. Gundlach et al., xe2x80x9cPentacene organic thin film transistorsxe2x80x94molecular ordering and mobilityxe2x80x9d, IEEE Electron Device Lett., Vol. 18, p. 87 (1997), to oligomers such as sexithiophenes or their variants, see for example reference F. Gamier et al., xe2x80x9cMolecular engineering of organic semiconductors: Design of self-assembly properties in conjugated thiophene oligomersxe2x80x9d, Amer. Chem. Soc., Vol. 115, p. 8716 (1993), polythiophenes of which poly(3-alkylthiophene), see for example reference Z. Bao et al., xe2x80x9cSoluble and processable regioregular poly(3-hexylthiophene) for field-effect thin film transistor application with high mobilityxe2x80x9d, Appl. Phys. Lett. Vol. 69, p4108 (1996), have been most studied. Although organic material-based TFTs generally provide lower performance characteristics than their conventional silicon counterparts, such as silicon crystal or polysilicon TFTs, they are nonetheless sufficiently useful for applications in areas where high mobility is not required. These include large-area devices, such as image sensors, active matrix liquid crystal displays and low-end microelectronics such as smart cards and RFID tags. TFTs fabricated from organic or polymer materials may be functionally and structurally more desirable than conventional silicon technology in the aforementioned areas in that they may offer mechanical durability, structural flexibility, and the potential of being able to be incorporated directly onto the active media of the devices, thus enhancing device compactness for transportability. However, most small molecule or oligomer-based devices rely on difficult vacuum deposition techniques for fabrication. Vacuum deposition is selected because the small molecular materials are either insoluble or their solution processing by spin coating, solution casting, stamp printing do not generally provide uniform thin films. In addition, vacuum deposition may also have the difficulty of achieving consistent thin film quality for large area format. Polymer TFTs, such as those fabricated from regioregular polythiophenes of, for example, regioregular poly(3-alkylthiophene-2,5-diyl) by solution processes, while offering reasonably high mobility, suffer from their propensity towards oxidative doping in air. For practical low-cost TFT design, it is therefore essential to have a semiconductor material that is both stable and solution processable, and where its performance is not adversely affected by ambient oxygen, for example, regioregular polythiophenes such as poly(3-alkylthiophene-2,5-diyl) are very sensitive to air. The TFTs fabricated from these materials in ambient conditions generally exhibit very large off-current, very low current on/off ratios, and their performance characteristics degrade rapidly.
References that may be of interest include U.S. Pat. Nos. 6,150,191; 6,107,117; 5,969,376; 5,619,357, and 5,777,070.
Illustrated in FIGS. 1 to 4 are various representative embodiments of the present invention and wherein polythiophenes are selected as the channel materials in thin film transistor (TFT) configurations.
It is a feature of the present invention to provide semiconductor polymers, such as polythiophenes, which are useful for microelectronic device applications like thin film transistor devices.
It is another feature of the present invention to provide polythiophenes with a band gap of from about 1.5 eV to about 3 eV as determined from the absorption spectra of thin films thereof, and which polythiophenes are suitable for use as thin film transistor semiconductor channel layer materials.
In yet a further feature of the present invention there are provided polythiophenes which are useful as microelectronic components, and which polythiophenes have reasonable solubility of, for example, at least about 0.1 percent by weight in common organic solvents, such as methylene chloride, tetrahydrofuran, toluene, xylene, mesitylene, chlorobenzene, and the like, and thus can be economically fabricated by solution processes, such as spin coating, screen printing, stamp printing, dip coating, solution casting, jet printing and the like.
Another feature of the present invention resides in providing electronic devices, such as thin film transistors with a polythiophene channel layer, and which layer has a conductivity of from 10xe2x88x926 to about 10xe2x88x929 S/cm (Siemens/centimeter).
Also, in yet another feature of the present invention there are provided polythiophenes and devices thereof, and which devices exhibit enhanced resistance to the adverse effects of oxygen, that is, these devices exhibit relatively high current on/off ratios, and their performance does not usually degrade as rapidly or minimal degradation results as those fabricated from regioregular polythiophenes such as regioregular poly(3-alkylthiophene-2,5-diyl).
Additionally, in a further feature of the present invention there is provided a class of polythiophenes with unique structural features which are conducive to molecular self-alignment under appropriate processing conditions, and which structural features also enhance the stability of device performance. Proper molecular alignment can result in higher molecular structural order in thin films, permitting efficient charge carrier transport, and thus higher electrical performance.
Aspects of the present invention include polythiophenes of the formula 
wherein R and Rxe2x80x2 are side chains; A is a divalent linkage; x and y represent the number of unsubstituted thienylene units or segments; z is 0 or 1, and wherein the sum of x and y is greater than zero; m represents the number of segments; and n represents the degree of polymerization; a polythiophene wherein the side chains are independently selected from the group consisting of alkyl, alkyl derivatives of alkoxyalkyl; siloxy-substituted alkyl, perhaloalkyl and polyether; A is an alkylene or arylene optionally of phenylene, biphenylene, phenanthrenylene, dihydrophenanthrenylene, fluorenylene, oligoarylene, methylene, polymethylene, dialkylmethylene, dioxyalkylene, dioxyarylene, or oligoethylene oxide; and n is from about 5 to about 5,000; a polythiophene wherein the number average molecular weight (Mn) of the polythiophenes is from about 2,000 to about 100,000 and the weight average molecular weight (Mw) is from about 4,000 to about 500,000, both as measured by gel permeation chromatography using polystyrene standards; a polythiophene wherein the polythiophene is (1), (2), (3), (4), (5), or (6) 
a polythiophene represented by (1), (2), or (3) 
a polythiophene represented by 
polythiophene (I) wherein x and y are integers of from zero to about 10, z is zero or 1, and m is from 1 to about 5; a polythiophene wherein x and y are integers of from about 1 to about 7, z is zero or 1, m is from 1 to about 5, and n is from about 5 to about 3,000; polythiophene (I) wherein the polyhaloalkyl is a perfluoroalkyl; polythiophene (I) wherein Mw is from about 5,000 to about 100,000, Mn is from about 4,000 to about 50,000; the side chain is alkyl with from about 3 to about 25 carbon atoms, or alkoxy with from 3 to about 25 carbon atoms, and A is an arylene; a polythiophene wherein the R and Rxe2x80x2 contain from about 3 to about 20 carbon atoms; polythiophene (I) wherein R and Rxe2x80x2 are independently selected from the group consisting of alkyl, alkyl derivatives of alkoxyalkyl; siloxy-substituted alkyl, perhaloalkyl of perfluoroalkyl and polyether; A is selected from the group consisting of arylene of phenylene, biphenylene, phenanthrenylene, dihydrophenanthrenylene, fluorenylene, dioxyalkylene, and dioxyarylene; polythiophene (I) wherein the R and Rxe2x80x2 are independently selected from the group consisting of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and isomers thereof; polythiophene (1) wherein R and Rxe2x80x2 are selected from the group consisting of hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, and pentadecyl; A is selected from the group consisting of phenylene, biphenylene, and fluorenylene; x and y are each independently a number of from zero to about 10; and m is a number of from 1 to about 5; polythiophene (I) wherein n is from about 7 to about 5,000; the number average molecular weight (Mn) of the polythiophene is from about 2,000 to about 100,000; weight average molecular weight (Mw) is from about 4,000 to about 500,000, both as measured by gel permeation chromatography using polystyrene standards; a polythiophene wherein A is phenylene, biphenylene, or fluorenylene; a polythiophene wherein n is from about 5 to about 5,000; a polythiophene wherein n is from about 10 to about 1,000; a polythiophene wherein the polythiophene is selected from the group consisting of polythiophenes of Formulas (1) through (8) 
polythiophene (I) wherein x, y and m are from 1 to 3, and z is 0 or 1; polythiophene (I) wherein x, y and m are 1, and z is 0 or 1; polythiophene (I) wherein Mn is from about 4,000 to about 50,000, and Mw is from about 5,000 to about 100,000; a polythiophene wherein the sum of x and y is from about 1 to about 10; a polythiophene wherein the sum of x and y is from about 1 to about 3; a polythiophene wherein n is from about 10 to about 4,000, and m is from about 1 to about 5; and polythiophenes encompassed by 
wherein R and Rxe2x80x2 are side chains independently selected, for example, from the group consisting of alkyl, alkyl derivatives, such as alkoxyalkyl; siloxy-substituted alkyl, perhaloalkyl, such as perfluoroalkyl, polyether, such as oligoethylene oxide, polysiloxy, and the like; A is a divalent linkage selected, for example, from the group consisting of arylene, such as phenylene, biphenylene, phenanthrenylene, dihydrophenanthrenylene, fluorenylene, oligoarylene, methylene, polymethylene, dialkylmethylene, dioxyalkylene, dioxyarylene, oligoethylene oxide, and the like; x and y are integers independently selected from 0 to about 10, z is either 0 or 1 with the provision that the sum of x and y is greater than zero; m is an integer of from 1 to about 5; and n is the degree of polymerization, and can generally be from about 5 to over 5,000, and more specifically, from about 10 to about 1,000. The number average molecular weight (Mn) of the polythiophenes can be, for example, from about 2,000 to about 100,000, and more specifically, from about 4,000 to about 50,000, and the weight average molecular weight (Mw) thereof can be from about 4,000 to about 500,000, and more specifically, from about 5,000 to about 100,000 both as measured by gel permeation chromatography using polystyrene standards.
Examples of the side chains R and Rxe2x80x2 include alkyl with, for example, from about 1 to about 25, and more specifically, from about 4 to about 12 carbon atoms, such as butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, isomeric forms thereof, and the like; alkoxyalkyl with from 1 to about 25 carbon atoms, such as for example methoxypropyl, methoxybutyl, methoxyhexyl, methoxyhexyl, methoxyheptyl, and the like, polyether chains, such as polyethylene oxide, perhaloalkyl, such as perfluoroalkyl of, for example, nonafluorohexyl, nonafluoroheptyl, pentadecafluorooctyl, tridecafluorononyl, and the like, and a polysiloxy chain, such as trialkylsiloxyalkyl, and the like.
More specifically, examples of thiophenes are 
The polythiophenes, reference Formula (I), are comprised of regioregular segments of 3,4-disubstituted-2,5-thienylene units, unsubstituted 2,5-thienylene units, and an optional divalent linkage. The regioregularity of the side chains in polythiophene (I), reference Formulas (1) through (14), is believed to be capable of inducing molecular self-alignment during thin film fabrication under appropriate processing conditions enabling highly organized microstructures in thin films. Higher order microstructures in a semiconductor channel layer of a thin film transistor enhance transistor performance. It is believed that these polythiophenes, when fabricated as thin films of about 10 nanometers to about 500 nanometers from their solutions in appropriate solvent systems, form strong intermolecular xcfx80-xcfx80 stacks which are conducive to efficient charge carrier transport. The unsubstituted thienylene moieties in (I), by virtue of possessing some degree of rotational freedom, help to disrupt the extended intramolecular xcfx80-conjugation of (I) to an extent that is sufficient to suppress its propensity towards oxidative doping. Accordingly, the polythiophenes are stable in ambient conditions, and the devices fabricated from these polythiophenes are functionally more stable than regioregular polythiophenes such as regioregular poly(3-alkylthiophene-2,5-diyl). When unprotected, the devices fabricated from Formula (I) polythiophenes in embodiments thereof are generally stable for weeks or even months, such as for example about 3 weeks to about 12 weeks rather than days, such as for example about less than 5 days for the devices of regioregular poly(3-alkylthiophene-2,5-diyl) when exposed to ambient oxygen; also the devices fabricated from the polythiophenes provide higher current on/off ratios, and their performance does not change as rapidly as those of poly(3-alkylthiophene-2,5-diyl) when no rigorous procedural precautions are taken to exclude ambient oxygen during material preparation, device fabrication, and evaluation. The materials stability against oxidative doping is particularly useful for low-cost device manufacturing; since the materials are more stable, they usually do not have to be handled in a strictly inert atmosphere and the processes of preparation are, therefore, simpler and more cost effective, and which processes are amenable to simple large-scale production processes.
The polythiophenes in embodiments are soluble in common coating solvents; for example, they possess a solubility of at least about 0.1 percent by weight, and more specifically, from about 0.5 percent to about 15 percent by weight in such solvents as methylene chloride, 1,2-dichloroethane, tetrahydrofuran, toluene, xylene, mesitylene, chlorobenzene, and the like. Moreover, the polythiophenes when fabricated as semiconductor channel layers in thin film transistor devices provide a stable conductivity of, for example, from about 10xe2x88x929 S/cm to about 10xe2x88x926 S/cm, and more specifically, from about 10xe2x88x928 S/cm to about 10xe2x88x927 S/cm as determined by conventional four-probe conductivity measurement.
The polythiophenes of the present invention can be prepared by polymerization of a properly constructed monomer, such as for example a trithiophene monomer, 2,5-bis(2-thienyl)-3,4-di-R-thiophene (IIa), or 2,5-bis(5-bromo-2-thienyl)-3,4-di-R-thiophene (IIb) for the preparation of illustrative polythiophenes (Ia) and (lb) according to Scheme 1. As the monomers ((IIa) and (IIb) carry two side chains on their respectively central thienylene units, their polymerizations, therefore, lead to polythiophenes (Ia) and (Ib) whose side chains are regioregularly positioned on their respective polythiophene backbones. Unlike the preparation of regioregular polythiophenes, such as poly(3-alkylthiophene-2,5-diyl) which require regioregular coupling reaction, the polythiophenes of the present invention can be prepared by general polymerization techniques without regioregularity complications. Specifically, (Ia) can be prepared from monomer (IIa) by FeCl3-mediated oxidative coupling polymerization or from monomer (IIb) by treating with Reike zinc, followed by addition of Ni(dppe)Cl2 catalyst. Polythiophene (IIb), on the other hand, can be readily obtained from (IIb) by Suzuki coupling reaction with appropriate arylene diboronate. 
Specifically, the polymerization of (IIa) can be accomplished by adding a solution of 1 molar equivalent of (IIa) in a chlorinated solvent, such as chloroform, to a suspension of about 1 to about 5 molar equivalent of anhydrous FeCl3 in chloroform under a blanket of dried air. The resultant mixture is allowed to react at a temperature of about 25xc2x0 C. to about 50xc2x0 C. under a blanket of dried air or with a slow stream of dried air bubbling through the reaction mixture for a period of about 30 minutes to about 48 hours. After the reaction, the polymer product is isolated by washing the reaction mixture with water or dilute aqueous hydrochloric acid solution, stirring with dilute aqueous ammonium solution, followed by washing with water, and then precipitated from methanol or acetone. For the Reike zinc method, 10 mmolar equivalents of (IIb) in anhydrous tetrahydrofuran is added dropwise over a period of 20 minutes to 40 minutes to a well-stirred suspension of 11 mmolar equivalent of freshly prepared Reike Zn in anhydrous tetrahydrofuran, and the resulting mixture is then permitted to react for about 30 minutes to about 2 hours at room temperature, about 22xc2x0 C. to about 25xc2x0 C. Subsequently, a suspension of about 0.1 mmolar equivalent of Ni(dppe)Cl2 in anhydrous tetrahydrofuran is slowly added over a period of about 10 minutes to about 20 minutes, and the mixture is then heated at about 40xc2x0 C. to about 65xc2x0 C. for 2 to 5 hours. The reaction mixture is then poured into dilute hydrochloric acid solution in methanol with vigorous stirring to precipitate the polymer product. The latter is redissolved in hot tetrahydrofuran and then reprecipitated from dilute ammonia solution in methanol.
More specifically, polythiophene (Ib) can be obtained by the Suzuki coupling reaction of monomer (IIb) with an appropriate arylene-diboronate. A mixture of equal molar equivalents of (IIa) and arylene-diboronate in toluene, about 2 to 6 molar percent of tetrakis(triphenylphosphine)-palladum, about 2 to about 4 molar equivalents of an inorganic base, such as sodium carbonate, in the form of a 1 M to 2 M aqueous solution, and about 1 to 5 mole percent of a phase transfer catalyst, such as tetrabutylamomonium chloride or tricaprylylmethylammonium chloride, is heated at about 90xc2x0 C. under an inert atmosphere for 48 hours. After polymerization, polythiophene product (Ib) is isolated by repeated precipitation from methonol.